Low-energy limit of the three-band model for electrons in a CuO$_{2}$ plane

The three-band model with the O-O direct hopping near to unit filling is considered. We present the general procedure of reduction of this model to the low-energy limit. At unit filling the three-band model in the charge-transfer limit is reduced to the Heisenberg model and we calculate the superexchange constant. For the case of the small electron doping the three-band model is reduced to the $t-J$ model and we calculate electron hopping parameters at the nearest and next neighbors. We derive the structure of corrections to the $t-J$ model and calculate their magnitude. The values of the hopping parameters for electron- and hole-doping differ approximately at 40 %.